How many of you struggle with fuel as your gauge does not work? You fill up, note the mileage then fill up again at 200 miles and hope for the best. Actually, the fuel gauge is almost certainly working, but the sender unit is at fault.
If you put a test meter on the sender, you should see about 73 ohms (Beetle and late bay) or 100 ohms (early bus with a balance coil gauge) across the rheostat when the tank is completely empty. As the tank is filled, the heater receives more current, moving the needle upwards on the gauge / dial until the tank is full sending the most current to the gauge at which point the tester should ready about 10 ohms.
At the back of your dashboard you will find the wiring to the speedo. On top of the fuel tank behind the firewall, behind the engine, you will find the sender.
Where is the fault?
The simplest check to find the fault is to remove the wire that is NOT brown from the top of the sender. The brown one is an earth on a late bay and an early bay only has a single wire. The gauge in the dashboard should go immediately to empty. Take that same wire and earth it (but not to the tank). Now the gauge should immediately jump to full.
If you have a brown wire, double check that it does actually go to earth and makes a good connection.
If the gauge went to empty and full during testing and the earth is good then the sender is faulty and can be replaced from the normal stockists. Bad news is that the early bay ones are about £60 and late bay ones are about £30.
If the issue is intermittent, tighten the hex bolts on the back of the gauge and check the fuse supplying this circuit has completely clean contacts and good cable.